Younger siblings of children with autism spectrum disorders had a greater risk of being affected with an ASD themselves, a Danish study affirmed.

Action Points

Note that this Danish population study demonstrated an elevated risk for autism spectrum disorders (ASDs) among younger siblings of children with ASDs.

Be aware that the elevated risk among maternal half-siblings versus paternal half-siblings suggests that pregnancy or intrauterine conditions may predispose to ASD.

Younger siblings of children with autism spectrum disorders (ASDs) had a greater risk of being affected with an ASD themselves, a Danish study affirmed.

Compared with children who came from families unaffected by ASDs, those with an older sibling with an ASD were nearly seven times more likely to be diagnosed with an ASD (HR 6.9, 95% CI 6.1-7.8), according to Therese Gronberg, MSc, of Aarhus University in Denmark, and colleagues.

The risk was significantly greater for full siblings than for half-siblings, the researchers reported online in JAMA Pediatrics.

The magnitude of the risk of an ASD recurrence within a family, however, was lower than that seen in recent studies using clinic-based populations. The recurrence risk was 6.1% overall in the current study, compared with 14.2% and 18.7% in studies published in the last few years.

"If used as a benchmark for parental counseling, this estimate [6.1%] should be reassuring to parents having a child with an ASD if they are considering having another child," Grønberg and colleagues wrote.

Stefani Hines, MD, of Beaumont Children's Hospital in Royal Oak, Mich., told MedPage Today in an email that "this study gives professionals who provide care to children with ASD and their families another data point regarding ASD recurrence risk and the likely combination of genetics and environmental factors contributing to such risk."

Both environmental and genetic factors are believed to be involved in causing ASDs, but it's not clear what has caused the increase in reported cases. Proposed explanations include earlier diagnosis, changes in diagnostic criteria, improved identification of cases, greater awareness, or an actual increase in prevalence.

Previous studies have shown that siblings of children with an ASD diagnosis have a greater risk of developing autism -- supporting a genetic cause -- although data are limited.

To explore the issue, Grønberg and colleagues examined data from more than 1.5 million children who were born in Denmark from January 1980 through December 2004 and were followed through December 2010. All ASD diagnoses were made by child psychiatrists.

Overall, 1.2% of children received an ASD diagnosis, but recurrence risks in younger siblings were higher, ranging from 4.5% to 10.5% across the birth years.

After adjustment for parity, the child's sex, parental age at birth, and parental psychiatric history, children with an ASD-affected older sibling were more likely to receive an ASD diagnosis compared with other children. The magnitude of the risk did not change over the course of the study.

"Therefore, the factors contributing to the risk for ASDs recurrence in siblings (perhaps a combination of genetic and environmental factors) either have not changed over time, or the changes have not affected the risk for recurrence," the authors wrote.

The researchers divided the cohort into two groups consisting of mothers or fathers who had at least two children. The recurrence risk seen in the overall cohort was similar to that seen among the maternal and paternal full siblings.

However, the risk associated with having an older sibling with an ASD was significant only for maternal half-siblings (HR 2.4, 95% CI 1.4-4.1) and not for paternal half-siblings (HR 1.5, 95% CI 0.7-3.4).

"The authors discuss that the estimates reported by this study support a genetic influence to ASDs, but the elevated relative recurrence risk among maternal half-siblings when compared to paternal half-siblings [suggests] that there may be other perinatal factors that may contribute," commented Sarah Nyp, MD, of Children's Mercy Hospitals and Clinics in Kansas City, Mo.

Although the increased risk of ASD recurrence in children with older affected siblings could be related to increased parental awareness, Grønberg and colleagues noted, "a parental awareness effect may be mitigated somewhat in population-based studies like ours that include all families and void bias from differential participation."

They acknowledged that the study was limited by the inability to account for the effect of parents deciding to stop having children after having a child diagnosed with an ASD, and by the possibility of missing some ASD cases.

The study was funded by Aarhus University.

The authors reported that they had no conflicts of interest.

Reviewed by F. Perry Wilson, MD, MSCE Instructor of Medicine, Perelman School of Medicine at the University of Pennsylvania and Dorothy Caputo, MA, BSN, RN, Nurse Planner

Accessibility Statement

At MedPage Today, we are committed to ensuring that individuals with disabilities can access all of the content offered by MedPage Today through our website and other properties. If you are having trouble accessing www.medpagetoday.com, MedPageToday's mobile apps, please email legal@ziffdavis.com for assistance. Please put "ADA Inquiry" in the subject line of your email.